1. Field of the Invention
The present invention relates to nonreducing dielectric ceramics and monolithic ceramic capacitors provided with dielectric ceramic layers composed of the nonreducing dielectric ceramics. More particularly, the invention relates to monolithic ceramic capacitors which are advantageously used under the conditions of high-frequency AC or high to medium voltage DC and are provided with internal electrodes composed of base metals, and to nonreducing dielectric ceramics for constituting dielectric ceramic layers of the monolithic ceramic capacitors.
2. Description of the Related Art
A conventional monolithic capacitor is usually produced in a method described below.
First, a plurality of ceramic green sheets containing a dielectric material for forming dielectric ceramic layers are prepared, in which the surface of each ceramic green sheet is coated with an electrode material for forming internal electrodes. As the dielectric material, for example, a material containing BaTiO.sub.3 as a principal constituent is used. The plurality of ceramic green sheets each coated with the electrode material are laminated and are subjected to thermal compression, and the resultant compact is fired, and thus a ceramic laminate provided with internal electrodes is obtained. By fixing and baking external electrodes that electrically connect to the internal electrodes on the sides of the ceramic laminate, a monolithic ceramic capacitor is obtained.
Accordingly, as the material for such internal electrodes, typically, a material which is not oxidized by the firing of ceramic laminates has been selected. Examples of such a material include noble metals such as platinum, gold, palladium, and silver-palladium alloys. However, although such materials for internal electrodes have excellent characteristics, they are very expensive, resulting in an increase in production cost of monolithic ceramic capacitors.
In order to reduce the production cost, monolithic ceramic capacitors provided with internal electrodes composed of base metals such as nickel and copper which are relatively inexpensive have been disclosed.
However, the base metals are easily oxidized in high-temperature, oxidizing atmospheres, which ruins the function as the internal electrodes. In order to use base metals as materials for internal electrodes of monolithic ceramic capacitors, firing must be performed in a neutral or reducing atmosphere to obtain ceramic laminates. On the other hand, if firing is performed under a low oxygen partial pressure, such as in a neutral or reducing atmosphere, the ceramic constituting dielectric ceramic layers is significantly reduced and becomes semiconductive.
Therefore, dielectric ceramics which do not become semiconductive even if firing is performed under a low oxygen partial pressure in order to avoid the oxidation of base metals have been disclosed. For example, Japanese Examined Patent Publication No. 61-14611 discloses a BaTiO.sub.3 --(Mg, Zn, Sr, Ca)O--B.sub.2 O.sub.3 --SiO.sub.2 -based dielectric ceramic, and Japanese Unexamined Patent Publication No. 7-272971 discloses a (Ba, M, L)(Ti, R)O.sub.3 -based dielectric ceramic (where M is Mg or Zn, L is Ca or Sr, and R is Sc, Y or a rare-earth element).
With the recent development of electronic devices which are highly integrated, have sophisticated functions, and are inexpensive, the characteristics required for use of monolithic ceramic capacitors have become severer. There is now a great demand for lower loss, better insulating properties, higher dielectric strength, higher reliability, larger capacity, lower prices, etc. with respect to monolithic ceramic capacitors.
Recently, the demand is also increasing for monolithic ceramic capacitors which can be used at high frequencies and high voltages or high currents. In such a case, small loss and low heat emission are important characteristics required for monolithic ceramic capacitors. The reason for this is that if a monolithic ceramic capacitor has large loss and high heat emission, the life of the monolithic ceramic capacitor itself is shortened. Due to the loss and heat emission of the monolithic ceramic capacitor, the temperature in circuits is also increased, resulting in operation errors and shortening of life with respect to peripheral components.
Moreover, monolithic ceramic capacitors are increasingly used under high-voltage DC conditions. However, since the conventional monolithic ceramic capacitors which particularly uses nickel as the material for internal electrodes are intended to be used at a relatively low electric field intensity, if they are used at high electric field intensity, insulating properties, dielectric strength and reliability are greatly deteriorated, which is disadvantageous.
When monolithic ceramic capacitors are produced using dielectric ceramics disclosed in Japanese Examined Patent Publication No. 61-14611 and Japanese Unexamined Patent Publication No. 7-272971, although the temperature-dependent variation in the capacitance is small, the loss and heat emission are increased during use at high frequencies and high voltages or high currents. Since such dielectric ceramics are nonreducing, it is possible to use base metals such as nickel as the material for internal electrodes by firing under a low oxygen partial pressure. However, when monolithic ceramic capacitors obtained by such firing under a low oxygen partial pressure are used under high-voltage DC conditions, insulation resistance is decreased and reliability is also decreased, which is disadvantageous.